1. Field of the Invention
The present invention generally relates to a semiconductor device including a pseudo lattice matched layer formed with a lattice mismatched semiconductor crystal. More particularly, the present invention relates to electronic devices such as: a light emitting devices, for example a surface emitting semiconductor laser and an edge emitting semiconductor laser used as a light source of a laser printer, a DVD device or a display; light receiving devices, for example a solar cell and a light quantity measuring sensor; and a transistor and so on.
2. Description of the Prior Art
Conventionally, layers constituting a semiconductor laser have basically been formed by stacking lattice matched materials except for a thin quantum well layer, a thin barrier layer and so on. For example, in a case of an AlGaAs based laser, AlAs (AlGaAs) lattice matched to and having a bandgap larger than GaAs and GaAs are employed to form heterojunctions and produce a band structure including a valence band and a conduction band of a laser. Further, in a case of GaInP based laser, crystals of Ga0.51In0.49P and (AlGa)0.51In0.49P whose bandgap is larger than that of Ga0.51In0.49P are employed to form heterojunctions and produce a band structure of a laser, wherein the crystals are both lattice matched to GaAs.
In a case of a nitride semiconductor that has recently drawn attention as a material for a blue semiconductor laser, there is almost no material lattice matched to GaN that is used as a substrate. Further, a mixed crystal of Al0.18In0.82N, which is one of a few lattice matched materials, has a problem because of a bandgap smaller than GaN and poor crystallinity, whereby there has been found almost no way for the mixed crystal to be used as a device constituent layer. That is, in the case of a nitride semiconductor material, neither an edge emitting laser nor a surface emitting laser has been able to be produced from a lattice matched material Hence, lattice mismatched materials have been used for the purpose despite easy occurrence of crystal defects or the like.
For example, a DBR mirror of a surface emitting semiconductor laser has been formed with a multilayer having lattice mismatched materials (AlxGa1xe2x88x92xN/GaN) (see Japanese Journal of Applied Physics Vol. 37 (1998), pp. L1412 to L1426). However, in the multilayer, if a composition x of AlxGa1xe2x88x92xN is set as high as Al0.34Ga0.66N, crystal defects such as cracks and misfit dislocations are produced because of a large difference in in-plane lattice constant between AlxGa1xe2x88x92xN and GaN, leading to poor crystallinity. Further, if a composition x of AlxGa1xe2x88x92xN is set low, though the lattice constants are closer to each other, a difference in refractive index between AlGaN and GaN is small, thereby reducing a reflectance of light and narrowing a wavelength range of reflection, which are both problematic.
There has also been proposed a nitride semiconductor laser in which a DBR mirror of a surface emitting semiconductor laser is constituted of a multilayer having dielectric different from the nitride semiconductor (see JP 98-308558 A). In this case, since a nitride semiconductor cannot directly be grown on a dielectric DBR mirror, the dielectric DBR is patterned by etching or the like such that part of a GaN crystal is exposed and a crystal of the nitride semiconductor is then laterally grown over the dielectric DBR starting from the exposed area of the GaN crystal, whereby a spacer layer, an active region and so on of the surface emitting semiconductor laser are formed. However, in this method, a cavity (resonator) including a spacer layer, active layer and so on formed on a dielectric DBR mirror has to be grown in a lateral direction over a length equal or more than 10 to 20 xcexcm while restricting a thickness of the cavity within 3 to 4xcex (equal to or less than about 600 nm, wherein xcex is a wavelength). That is, as a result, a very thin flat crystal having opposite major surfaces parallel to each other is to be formed, thereby making fabrication of the laser difficult. Further, since this lateral growth is generally a case of a facet growth in which a crystallographic plane is preferentially grown, there is a fault that the growth speed is slow: the lateral growth over 10 xcexcm or more requires a long time, which makes the method not suitable for mass fabrication.
Also in a case of a surface emitting semiconductor laser of a long wavelength range such as for use in communications, a problem exists since there are few kinds of material lattice matched InP and a DBR mirror formed with an AlGaInAsP based material lattice matched to InP cannot have a high refractive index, thereby making it impossible to fabricate a practical surface emitting semiconductor laser. Hence, conventionally, a surface emitting semiconductor laser has been fabricated, for example, in such a way that a multilayer DBR mirror of AlAs/GaAs, which is a GaAs based material, having a good reflection characteristic is separately prepared and an active region layer of a long wavelength AlGaInAsP based laser is stuck on the mirror; since in-plane lattice constants are largely different from each other, direct growth cannot be performed. The fabrication process, though possible, is problematically cumbersome, making it impractical.
Further, while there can be exemplified a nitride semiconductor laser in which the cladding layers of an edge emitting semiconductor laser are fabricated with a lattice mismatched materials (AlxGa1xe2x88x92xN/GaN), in this case, if a composition x of AlxGa1xe2x88x92xN is set high, crystal defects such as cracks and misfit dislocations are produced and contrary to this, if a composition x of AlxGa1xe2x88x92xN is set low, there also arises a problem since optical confinement is insufficient and in turn laser beam is leaked to the substrate side, followed by other inconveniences.
While as described above, in a case of the nitride semiconductor, a semiconductor multilayer has been formed using a lattice mismatched material with GaN, there has been arisen a problem, since crystal defects such as cracks and misfit dislocations are produced due to a difference in lattice constant, thereby making it difficult to fabricate a semiconductor multilayer of good crystallinity. Further, there has arisen a problem since if a material of a lattice constant closer to that of GaN is used for forming a semiconductor multilayer on GaN, a reflection characteristic of the semiconductor multilayer is degraded, which in turn makes it impossible to use the semiconductor multilayer as a DBR mirror of a surface emitting semiconductor laser or a cladding layer of an edge emitting semiconductor laser. Still further, there has arisen a problem in a case of InP that is used in a long wavelength range laser such as for use in communications as well, since a semiconductor multilayer formed using an AlGaInAsP based material having a lattice matching performance has a poor reflection characteristic and thereby, when the semiconductor multilayer is employed as a DBR mirror of a surface emitting semiconductor laser or a cladding layer of an edge emitting semiconductor laser, no sufficient light emission characteristic is achieved.
The present invention has been made in view of the above circumstances and accordingly provides a semiconductor device having a pseudo lattice matched layer of good crystallinity, formed with lattice mismatched materials. The present invention further provides a semiconductor device having a pseudo lattice matched layer of a good reflection characteristic, formed with lattice mismatched materials. The present invention still further provides a good surface emitting semiconductor laser and a good edge emitting semiconductor laser.
According to one aspect of the present invention, a semiconductor device includes: a semiconductor base layer made of a semiconductor crystal whose in-plane lattice constant is a0; a pseudo lattice matched layer including a first layer made of a semiconductor crystal whose in-plane lattice constant is larger than a0 and a second layer made of a semiconductor crystal whose in-plane lattice constant is smaller than a0, the pseudo lattice matched layer being formed while being subjected to pseudo lattice matching by epitaxially growing the first and second layers on the semiconductor base layer such that lattice strains produced in the first and second layers assume opposite directions; and a functioning layer having at least one function of recombination, generation and migration of carriers.
In the present invention, a pseudo lattice matched layer including a first layer made of a semiconductor crystal whose in-plane lattice constant is larger than a0 and a second layer made of a semiconductor crystal whose in-plane lattice constant is smaller than a0 is epitaxially grown on the semiconductor base layer made of a semiconductor crystal whose in-plane lattice constant a0. That is, the pseudo lattice matched layer is epitaxially grown on the semiconductor base layer using lattice mismatched materials.
In a case where the first layer is comparatively thin (equal to or less than a critical thickness), the first layer is within the range of elastic deformation and there is no generation of misfit dislocations therein, but since a semiconductor crystal whose in-plane lattice constant is larger than a0 is epitaxially grown on the semiconductor base layer, a compressive stress remains in the grown semiconductor crystal. On the other hand, in a case where the second layer is comparatively thin (equal to or less than a critical thickness), the second layer is within the range of elastic deformation and there is no generation of misfit dislocations therein, but since a semiconductor crystal whose in-plane lattice constant is smaller than a0 is epitaxially grown on the semiconductor base layer, a tensile stress remains in the grown semiconductor crystal. Since the compressive and tensile stresses act in opposite directions, the stresses are canceled as a whole and therefore, no increase in residual stress occurs even when layer stacking is repeated many times, thereby enabling a pseudo lattice matched semiconductor layer to be formed.
That is, since semiconductor crystals respectively constituting the first and second layers, which are pseudo lattice matched layers, mutually relax stresses therein, the semiconductor crystals whose lattice constants are largely different from that of a semiconductor base layer can be epitaxially grown on the semiconductor base layer while effecting pseudo lattice matching. As a result, no crystal defects such as cracks and misfit dislocations are generated, thereby entailing achievement of the pseudo lattice matched semiconductor layer with good crystallinity.
A functioning layer provided in addition to the semiconductor base layer and the pseudo lattice matched layer acts one function of recombination, generation and migration of carriers. That is, a semiconductor device of the present invention works as a light emitting device when the semiconductor device is imparted with a carrier recombination function, a light receiving device when the semiconductor device is imparted with a carrier generation function and an electronic device when the semiconductor device is imparted with a carrier migration function.
According to another aspect of the present invention, an in-plane lattice constant ax of the pseudo lattice matched layer satisfies the relation described below in connection with the in-plane lattice constant a0 of the semiconductor crystal of the semiconductor base layer:
a0xc3x970.997xe2x89xa6axxe2x89xa6a0xc3x971.003
Since the pseudo lattice matched layer is a mixed crystal having plural semiconductor crystals with different lattice constants, a lattice constant x0 of the multilayer is given by the following formula, given that the number of layers respectively corresponding to the plural semiconductor crystals of the pseudo lattice matched layer is n:       a    x    =                    ∑                  i          =          1                n            ⁢              xe2x80x83            ⁢                        a          i                ⁢                  d          i                                    ∑                  i          =          1                n            ⁢              xe2x80x83            ⁢              d        i            
where ai and di are a lattice constant and a thickness of a semiconductor crystal in the i-th layer respectively.
All that is needed is to adjust compositions and thicknesses of semiconductor crystals constituting the first and second layers such that the lattice constant ax of the pseudo lattice matched layer almost coincides with the in-plane lattice constant a0 of semiconductor crystals of the semiconductor base layer. To be concrete, as shown in the relation, the lattice constant ax of the pseudo lattice matched layer is preferably within a fluctuation width xc2x10.3% of the lattice constant a0. When the lattice constant ax of the pseudo lattice layer is in this range, a multilayer crystal of high quality by no means inferior to a lattice matched crystal can be fabricated. On the other hand, if the lattice constant ax fluctuates outside the fluctuation width xc2x10.3% of the lattice constant a0, generation of crystal defects such as dislocations becomes conspicuous and there arise a possibility of generating cracks. Further, since irregularities, concave and convex, of a crystal surface are great and crystallinity is deteriorated, a DBR mirror, which is fabricated with such a multilayer crystal, problematically comes to have a poor reflectance and a narrowed reflection range.
According to another aspect of the present invention, an in-plane lattice constant a1 of the semiconductor crystal of the first layer and an in-plane lattice constant a2 of the semiconductor crystal of the second layer satisfy the relation described below:
xe2x80x83a0xe2x89xa7a0xc3x971.003
a2xe2x89xa6a0xc3x970.997
According to another aspect of the present invention, an in-plane lattice constant a1 of the semiconductor crystal of the first layer and an in-plane lattice constant a2 of the semiconductor crystal of the second layer satisfy the relation described below:
a1xe2x89xa7a0xc3x971.006
a2xe2x89xa6a0xc3x970.994
As shown in the third and fourth aspects, when semiconductor crystals with in-plane lattice constants largely different from each other are stacked in combination, there can be fabricated a semiconductor crystal layer with a large or small bandgap, or with a high or low refractive index, which cannot be achieved from lattice matched crystals. That is, since a band structure including a conduction band and a valence band can be altered with comparative freedom, a degree of device design freedom is also raised.
According to another aspect of the present invention, the pseudo lattice matched layer is formed by alternately stacking the first and second layers.
According to another aspect of the present invention, the pseudo lattice matched layer includes a third layer made of a semiconductor crystal whose in-plane lattice constant is a0 in addition to the first and second layers.
According to another aspect of the present invention, at least one of the first and second layers is formed by stacking plural semiconductor crystals with different in-plane lattice constants.
According to another aspect of the present invention, each layer of the pseudo lattice matched layer has a thickness equal to or less than 10 nm.
The first and second layers included in the pseudo lattice matched layer are only required to be epitaxially grown on a semiconductor base layer such that stresses respectively generated in the first and second layers are relaxed and there can be various ways of stacking the layers, which are shown as typical examples shown in the fifth to eighth aspects of the present invention as representatives.
Especially, when each layer included in the pseudo lattice matched layer is thinned (preferably to a value equal to or less than 10 nm), that is, formed as a short period superlattice layer, then mini bands are formed within the pseudo lattice matched layer and a bandgap of the pseudo lattice matched layer becomes larger than that of a semiconductor crystal of the semiconductor base layer. The bandgap is derived from Kronig-Penny theoretical equation on mini band formation in a short period superlattice. The pseudo lattice matched layer of a large bandgap can be applied to cladding layers disposed on both sides of the active layer of an edge emitting semiconductor laser.
There is a problem, for example, in the case of GaN based materials that Al0.18In0.82N lattice matched to a GaN crystal has a bandgap smaller than GaN. When Al0.18In0.82N is replaced by a pseudo lattice matched crystal having a short-period supper lattice structure including AlGaN and GaInN, a bandgap of the pseudo lattice matched crystal can be made larger than that of GaN. As shown in FIG. 7, an AlInN mixed crystal has an extremely large bandgap bowing. However, a pseudo latticed matched crystal alternately stacked with AlxGa1xe2x88x92xN (x greater than 0) and Ga1xe2x88x92yInyN (y greater than 0) has a bandgap larger than Al0.18In0.82N.
According to another aspect of the present invention, a bandgap of the pseudo lattice matched layer is larger than a bandgap of the semiconductor crystal of the semiconductor base layer.
While semiconductor crystals having a strain of 0.3% relative to GaN, which is a semiconductor crystal of a semiconductor base layer, in GaN based materials include Al0.12Ga0.88N and Ga0.97In0.03N, the bandgap of Al0.12Ga0.88N is larger than that of GaN by about 250 meV and the bandgap of Ga0.97In0.03N is smaller than that of GaN by about 70 meV. When the semiconductor crystals are formed in a very short period superlattice structure, a bandgap of the short period superlattice is larger than GaN by about 90 meV. While semiconductor crystals each having an increased strain of 0.6% relative to GaN are Al0.24In0.76N and Ga0.94In0.06N, if the semiconductor crystals are formed in a very short period superlattice structure, a bandgap of the short period superlattice is larger than that of GaN by about 180 meV.
The differences of the bandgaps between the short period superlattices and GaN are magnitudes that can each sufficiently confine electrons with a heterojunction interface of a band structure, and further can sufficiently be used in formation of a potential well in a laser active layer or as a potential difference between the emitter/base of a hetero-bipolar transistor as well. In the present invention, such a band structure constituted of bandgaps different in magnitudes can be formed by growing good crystals while being subjected to pseudo lattice matching.
According to another aspect of the present invention, the pseudo lattice matched layer is formed by alternately stacking high and low refractive index regions.
For example, while semiconductor crystals having a strain of 0.3% relative to GaN, which is a semiconductor crystal of a semiconductor base layer, in GaN based materials includes Al0.12Ga0.88N and Ga0.97In0.03N, a refractive index of Al0.12Ga0.88N is smaller than that of GaN by about 0.07 and a refractive index of Ga0.97In0.03N is larger than that of GaN by about 0.1. Accordingly, when Al0.12Ga0.88N and Ga0.97In0.03N are alternately stacked, high refractive index layers and low refractive index layers are alternately formed in a pseudo lattice matched layer.
According to another aspect of the present invention, the high refractive index regions and the low refractive index regions are alternatively stacked with each region of mxcex/4 in thickness, where xcex is an emission wavelength from an active region and m=1 or 3, to form a distributed-Bragg reflector mirror.
While high refractive index regions and low refractive index regions are alternatively stacked with each region in stacking of mxcex/4 in thickness, where xcex is an emission wavelength from an active region and m=1 or 3, and thereby a distributed-Bragg reflecting mirror can be formed, since semiconductor crystals in a pseudo lattice matched layer are of high crystallinity, the semiconductor crystals can work as a distributed-Bragg reflecting mirror having a high reflection characteristic and a broad reflection range, which has not been achieved by a conventional lattice matched crystal. In order to improve a reflection characteristic, a refractive index difference xcex94n is preferably set to 0.3 or larger.
While, in a case of GaN based materials, semiconductor crystals that are strained relative to GaN by 0.6%, which is a semiconductor crystal constituting a semiconductor base layer, include Al0.24Ga0.76N and Ga0.94In0.06N, semiconductor crystals thus strained can exert a better reflection characteristic as a DBR mirror of a surface emitting semiconductor laser since a difference xcex94n in refractive index between thus strained semiconductor crystals is 0.34.
It should be appreciated that calculation of refractive indices were performed based on famous Kramers-Kronig theoretical equation and results of actual measurements (reference document: Electronics Letters, 1996 Vol. 32, No. 24, 2285).
According to another aspect of the present invention, the semiconductor base layer is made of GaN or AlGaInN lattice matched to GaN, the first layer is made of Ga1xe2x88x92xInxN, wherein 0 less than xxe2x89xa61, and the second layer is made of AlyGa1xe2x88x92yN wherein 0 less than yxe2x89xa61.
According to another aspect of the present invention, the semiconductor base layer is made of InP or AlGaInAsP lattice matched to InP, the first layer is made of AlxIn1xe2x88x92xAsyP1xe2x88x92y, wherein 0 less than xxe2x89xa61 and 0xe2x89xa6y less than 1, and the second layer is made of GaxIn1xe2x88x92xAsyP1xe2x88x92y, wherein 0xe2x89xa6x less than 1 and 0 less than yxe2x89xa61.
Lattice matched materials from which a DBR mirror with a good reflection characteristic cannot be fabricated are, for example, GaN based semiconductor materials that emit short wavelength light such as blue and InP based semiconductor materials that emit light of a long wavelength ranging from 1.3 xcexcm to 1.55 xcexcm, which is used in communications or the like. The present invention is especially useful in use of these materials.
For example, in a case of InP based materials, an InP crystal is used as a semiconductor base layer, AlGaInAsP whose in-plane lattice constant is larger than that of InP is used as the first layer and GaInAsP whose in-plane lattice constant is smaller than that of InP is used as the second layer. Such a combination can realize a multilayer of a pseudo lattice matching type whose refractive index difference is larger than that not realized with any lattice matched materials that have conventionally been available. That is, since a DBR mirror with a good reflection characteristic can be fabricated, it is possible to grow a DBR mirror and an active region on a substrate at a time, resulting in an InP based surface emitting semiconductor laser with a good emission characteristic.
According to another aspect of the present invention, the semiconductor base layer is made of GaAs or AlGaInP lattice matched to GaAs, the first layer is made of GayIn1xe2x88x92yP, wherein y less than 0.51, and the second layer is made of AlzIn1xe2x88x92zP, wherein z greater than 0.51.
Even in a case where a proper material that is lattice matched to a semiconductor crystal of a semiconductor base layer is available, use of a lattice mismatched material extends freedom of material selection, which in turn enables a pseudo lattice matching type multilayer having characteristics that have been impossible to achieve with conventional lattice matched materials, for example a high reflection characteristic.
According to another aspect of the present invention, a surface emitting semiconductor laser includes: a semiconductor base layer made of a semiconductor crystal whose in-plane intrinsic crystal lattice constant is a0; a first distributed-Bragg reflector mirror of a pseudo lattice matching type including a first layer made of a semiconductor crystal whose in-plane lattice constant is larger than a0 and a second layer made of a semiconductor crystal whose in-plane lattice constant is smaller than a0, the first distributed-Bragg reflector mirror of a pseudo lattice matching type being formed while being subjected to pseudo lattice matching by epitaxially growing the first and second layers on the semiconductor base layer such that lattice strains produced in the first and second layers assume opposite directions; an active region formed on the first distributed-Bragg reflector mirror, which performs recombination of carriers; and a second distributed-Bragg reflector mirror that sandwiches the active region with the first distributed-Bragg reflector mirror, to form a resonator mirror structure with the first and second distributed-Bragg reflector mirrors.
According to another aspect of the present invention, the second distributed-Bragg reflector mirror is a distributed-Bragg reflector mirror of a pseudo lattice matching type including: a first layer made of a semiconductor crystal whose in-plane lattice constant is larger than a0 and a second layer made of a semiconductor crystal whose in-plane lattice constant is smaller than a0, the second distributed-Bragg reflector mirror of a pseudo lattice matching type being formed while being subjected to pseudo lattice matching by epitaxially growing the first and second layers on the semiconductor base layer such that lattice strains produced in the first and second layers assume opposite directions.
According to another aspect of the present invention, an edge emitting semiconductor laser includes: a semiconductor base layer made of a semiconductor crystal whose in-plane lattice constant is a0; a first cladding layer of a pseudo lattice matching type including a first layer made of a semiconductor crystal whose in-plane lattice constant is larger than a0 and a second layer made of a semiconductor crystal whose in-plane lattice constant is smaller than a0, the first cladding layer of a pseudo lattice matching type being formed while being subjected to pseudo lattice matching by epitaxially growing of the first and second layers on the semiconductor base layer such that lattice strains produced in the first and second layers assume opposite directions; an active region formed on the first cladding layer, which performs recombination of carrier; a second cladding layer that sandwiches the active region with the first cladding layer, to achieve light confinement in the active region; and a pair of edge reflector mirrors disposed in an opposite manner to each other, which resonate light generated in the active region in a predetermined plane direction of the active region.
Since a DBR mirror of a surface emitting semiconductor laser or a cladding layer of an edge emitting semiconductor laser is constituted of a pseudo lattice matched layer that is formed with lattice mismatched materials, all of layers whose thickness are comparatively large except for an active region can be fabricated by means of crystal growth, which can realize a highly reliable, long life-time semiconductor laser having no crystal defects such as cracks and misfit dislocations.
It should be appreciated that the term xe2x80x9ca semiconductor base layerxe2x80x9d means a semiconductor substrate, or a semiconductor crystal layer formed on the substrate.